Inductive power transfer (IPT) systems are well known. An example of a typical IPT system is shown in FIG. 1 in which a power supply 1 which is typically provided in the form of a converter, energises a primary conductive path or track 2. The power supply typically generates a high frequency current along the track 2, which is usually a discrete coil, but may in other embodiments comprise a single loop. The magnetic field generated in the track 2 enables a pick up coil 3 of a pick up device 4 to be coupled inductively to the track. The pick up 4 includes control circuitry 5 which conditions power received so as to provide an appropriate power supply to load 6.
To generate the high frequency current in track 2, the operating frequency of switches in the power supply 1 is usually made to be equal to the actual system frequency. This makes the controller design for these power supplies complicated and costly. In practice, controlling the power available to a pick up 4 can be problematic, and in existing systems attempts to provide adequate control can result in unpredicted voltage and current overshoots during load transients or start up situations. These can damage the switching devices or other components in the system.